The development of middle rare earth element enrichments in freshwaters: weathering of phosphate minerals

Abstract

Many phosphatic minerals of biogenic, authigenic and diagenetic origin are distinguished by rare-earth element (REE) compositions, which have a well-developed middle rare-earth element (MREE) enrichment defined by a gradual increase in the shale-normalized ratios of REE toward the middle of the REE series (La to Lu). Also, the REE composition of dissolved matter in the waters of the Amazon, Fly and Sepik Rivers (as well as other river, lake and ground waters) have well-developed MREE enrichments. The development of MREE enrichments was studied through a series of leaching experiments using river-bank sediments from the Amazon, Fly, Sepik and Mississippi Rivers. A well-characterized phosphate mineral-rich shale with a pronounced MREE enrichment in its bulk composition was also leached in order to better assess the relationship between the weathering of phosphate minerals and the formation of MREE-enriched solutions. The two leaching solutions per sediment consisted of filtered lake water at pH 5 and the same water adjusted to pH 2. Leaching experiments with the four river sediments result in extensive fractionation whereby the REE compositions of the solutions develop MREE enrichments from starting sediments that show no MREE enrichments. The pH 2 leachates have much higher concentrations of the REE and more fully developed MREE enrichments than do the pH 5 leachates. The MREE enrichments at pH 2 usually form within an hour of starting the experiments. The phosphatic shale results show that there is preferential release of large amounts of MREE at pH 5, as well as at pH 2, demonstrating the weather ability of the phosphatic minerals at pHs of natural freshwaters. The Sepik River pH 5 experiment also shows significant MREE enrichment under near-neutral conditions. The development and retention of the MREE enrichments in the leaching solutions is accompanied by an increase in dissolved phosphate concentrations. Although the magnitudes of the MREE enrichment do not directly correlate with the percent of phosphate released, the experimental data suggest that the preferential dissolution of phosphatic minerals during weathering can result in extensive REE fractionation between bulk river sediment and water, leading to freshwaters with MREE enrichments.